1. Field of the Invention
The present invention relates to a light source that emits light, which is obtained by multiplexing light from a plurality of light sources. The present invention also relates to an optical tomography imaging apparatus, which is equipped with the light source, and obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement.
2. Description of the Related Art
Conventionally, optical tomography imaging apparatuses that utilize OCT measurement are employed to obtain tomographic images of living tissue. In an optical tomography imaging apparatus, a low coherence light beam emitted from a light source is divided into a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected by a measurement target when the measuring light beam is irradiated onto the measurement target, is multiplexed with the reference light beam. Tomographic images are obtained, based on the intensity of a coherent light beam obtained by multiplexing the reflected light beam and the reference light beam.
There are some optical tomography imaging apparatuses that utilize TD-OCT (Time Domain OCT) measurement. In TD-OCT measurement, the measuring position in the depth direction (hereinafter, referred to as “depth position”) within a measurement target is changed, by changing the optical path length of the reference light beam. Thereby, tomographic images can be obtained at different depth positions within measurement targets.
As another type of optical tomography imaging apparatus that can obtain tomographic images at high speed without changing the optical path of the reference light beam, optical tomography apparatuses that employ SD-OCT (Spectral Domain OCT) measurement have been proposed. In SD-OCT measurement, a wide band low coherence light beam is divided into a measuring light beam and a reference light beam by a Michelson interferometer. Then, the measuring light beam is irradiated onto a measurement target, and a reflected light beam, which is the measuring light beam reflected by the measurement target, is multiplexed with the reference light beam, to obtain a coherent light beam. Thereafter, the coherent light beam is decomposed into different frequency components. The channeled spectra of the decomposed coherent light beam undergoes Fourier analysis, and tomographic images are obtained without scanning in the depth direction.
In the aforementioned optical tomography imaging apparatuses, improvement of spatial resolution is an important objective. It is known that spatial resolution in the depth direction of measurement targets can be increased the wider the wavelength interval of a light source is. In addition, it is preferable for the light source to be a stable point light source, and for the spectrum, representing intensity distribution with respect to wavelengths of the emitted light, to approximate a graduated Gaussian distribution.
SLD's (Super Luminescent Diodes) are comparatively low cost, currently available point light sources that have wide wavelength intervals. However, if it is attempted to obtain a wavelength interval exceeding 100 nm with a single element, the spectrum of light emitted thereby becomes multi-peaked and unstable. It is necessary to narrow the wavelength interval to obtain a stable, single peaked spectrum.
Recently, optical tomography imaging apparatuses that employ a plurality of light sources having different wavelengths have been proposed. Japanese Unexamined Patent Publication No. 6 (1994)-165784 discloses an optical tomography imaging apparatus that simultaneously irradiates low coherence light from a plurality of light sources onto a measurement target, separates reflected light beams and reference light beams with filters having wavelength selectivity, and obtains coherent signals for each of the separated light beams.
Further, Japanese Unexamined Patent Publication No. 2002-214125 discloses a light source for optical tomography image measurement. This light source suppresses side lobes caused when multiplexing light emitted from a plurality of low coherence light sources having different wavelengths, due to the multiply peaked spectra thereof, by optimizing the parameters of each of the low coherence light sources.
Optical tomography imaging apparatuses such as those described above generally utilize light emitted from light source apparatuses by causing them to enter transmission single mode optical fibers. That is, multiplexed light beams obtained by multiplexing light from a plurality of light sources are also caused to enter optical fibers, and therefore it is necessary to match the optical axes of light beams emitted from each light source after being multiplexed. Due to this requirement, usable multiplexing means are limited, and conventionally, dichroic mirrors and half mirrors have been used. However, these multiplexing means have the following shortcomings, and improvements are desired.
First, multiplexing by dichroic mirrors will be described. FIG. 23 illustrates the configuration of a light source apparatus that employs three dichroic mirrors DM1, DM2, and DM3 to multiplex light emitted from four light sources SLD1, SLD2, SLD3, and SLD4. The light sources SLD1, SLD2, SLD3, and SLD4 each emit light having different central wavelengths. FIG. 24 is a graph that illustrates the spectra of light emitted by the light sources SLD1, through SLD4, and wavelength properties of the reflectance of the dichroic mirrors DM1, DM2, and DM3. Note that in FIG. 24 the graduation of the left vertical axis that represents intensity is different from that of the right vertical axis that represents reflectance. The cutoff wavelengths of the dichroic mirrors DM1, DM2, and DM3 are the intersection of the intensity curves of the light sources SLD1 and SLD2, the intersection of the intensity curves of the light sources SLD2 and SLD3, and the intersection of the intensity curves of the light sources SLD3 and SLD4, respectively.
In the configuration illustrated in FIG. 23, the light sources SLD2, SLD3, and SLD4 are provided such that the directions of light beams emitted therefrom are perpendicular to the direction of a light beam emitted from the light source SLD1. The dichroic mirrors DM1, DM2, and DM3 are provided at the intersection of the light beams emitted from the light sources SLD1 and SLD2, the intersection of the light beams emitted from the light sources SLD1 and SLD3, and the intersection of the light beams emitted from the light sources SLD1 and SLD4, respectively. The dichroic mirrors DM1, DM2, and DM3 are arranged so as to be disposed at a 45 degree angle with respect to the light beams emitted from all of the light sources SLD1, SLD2, SLD3, and SLD4. Light beams emitted from the light sources SLD1, through SLD4 are multiplexed by the dichroic mirrors DM1, DM2, and DM3. The obtained multiplexed light beam propagates along a single optical axis.
FIG. 25 is a graph that illustrates spectra of the multiplexed light beam obtained by multiplexing the light beams emitted from the light sources SLD1 through SLD4. In FIG. 25, the full width at half maximum w of the four light sources SLD1, SLD2, SLD3, and SLD4 are set to be 10 nm. Spectra are illustrated for each of a case in which intervals d between peak wavelengths of the four light sources SLD1 through SLD4 are set such that d/w=1.0, d/w=0.5, and d/w=2.
Multiplexing by dichroic mirrors as described above can reduce light loss due to the steep wavelength selectivity of the dichroic mirrors. However, the spectrum of the multiplexed light becomes multiple peaked, with a shape having concavities and convexities, as illustrated in FIG. 25. In the case that the spectrum is of a shape having concavities and convexities, the following problems occur.
In SD-OCT measurement, Fourier transform is administered on detected signals from a wave number space to a positional space, to generate signals that represent changes in reflectance over depth positions. FIG. 26A is a graph that illustrates the spectra of light beams emitted from light sources prior to multiplexing with broken lines and the spectrum of a multiplexed light beam obtained by dichroic. mirrors with a solid line, as a wave number function. FIG. 26B is a graph that illustrates the spectrum of the multiplexed light beam, on which Fourier transform has been administered, as a positional function. Note that the graduations of the horizontal axis of the graph of FIG. 26B are not equidistant. The spectrum of the multiplexed light beam is of a shape that has concavities and convexities, as illustrated in FIG. 26A. When Fourier transform is administered on such a spectrum, side lobes SL appear, as illustrated in FIG. 26B. In actual SD-OCT measurement, Fourier transform is administered on signals which are overlaps of OCT coherent signals on signals based on the spectrum of a measuring light beam. If side lobes such as the side lobes SL illustrated in FIG. 26B are present at this time, they appear to be the same as components that indicate that a reflective interface is present at a certain depth position. Therefore, the side lobes become noise with respect to reflection data, cause signals to become unclear, and deteriorate resolution.
The differences between the peaks and bottoms of the concavities and convexities of a spectrum are determined by the wavelength intervals and the central wavelength intervals of the light sources SLD1 through SLD4. If the central wavelength intervals are reduced, the differences between the peaks and bottoms decrease, thereby enabling reduction of the concavities and convexities of the spectrum. However, the wavelength intervals cannot be broadened, precluding achievement of the original objective.
In order to solve the above problem, insertion of filters having wavelength selectivity into the optical path of the multiplexed light beam to smooth the spectrum, may be considered. However, the wavelength properties of the filters will be complex, and therefore this solution is impractical.
Next, multiplexing by half mirrors will be described. FIG. 27 illustrates the configuration of a light source apparatus that employs three half mirrors HM1, HM2, and HM3 to multiplex light emitted from four light sources SLD1, SLD2, SLD3, and SLD4. The light sources SLD1, SLD2, SLD3, and SLD4 each emit light having different central wavelengths.
As illustrated in FIG. 27, the light sources SLD1 and SLD3 are provided such that the directions of light beams emitted therefrom are perpendicular to each other. The half mirror HM1, which is provided at the intersection of the light beams emitted from the light sources SLD1 and SLD3, and arranged to be disposed at a 45 degree angle with respect to both of the two light beams, multiplexes the two light beams. The multiplexed light beam enters the half mirror HM3. Similarly, the half mirror HM2 multiplexes light beams emitted from the light sources SLD2 and SLD4, and the multiplexed light beam enters the half mirror HM3. The multiplexed light beam, which has been multiplexed by the half mirror HM1, and the multiplexed light beam, which has been multiplexed by the half mirror HM2, are multiplexed by the half mirror HM3. The obtained multiplexed light beam propagates along a single optical axis.
FIG. 28A is a graph that illustrates the spectra of light beams emitted from the four light sources SLD1 through SLD4 prior to multiplexing with broken lines and the spectrum of the multiplexed light beam with a solid line, as a wave number function. The spectrum of the multiplexed light beam is smooth, as illustrated in FIG. 28A. When Fourier transform is administered on this spectrum, slide lobes do not appear, as illustrated in FIG. 28B.
However, the transmissivity of each half mirror is 50%, and therefore there is a problem that the amount of light loss is great. As the number of light sources to be multiplexed increases, the number of times that light beams emitted from the light sources pass through the half mirrors increases, and the amount of light which is ultimately usable decreases. For example, in the configuration illustrated in FIG. 27, light beams emitted from all of the light sources SLD1 through SLD4 pass through two half mirrors. Therefore, the ultimately usable amount of light is 25% of the light emitted from the light sources, which is extremely inefficient.